In current communication networks, the users increasingly desire higher data transmission rates. Transmission methods which can provide high transmission rates also in the subscriber line networks (access networks) of a communication network are, for example, methods which operate in accordance with the xDSL method. Using these transmission methods, the operators of communication networks can also offer their customers broadband connections, for example to the Internet, by means of which the subscribers can use numerous applications in an increasingly more simple and rapid manner. An example of such a data-intensive application for which high transmission rates are needed is, for example, the transmission of video files via the Internet.
Types of embodiments of the abovementioned xDSL method are, for example, the so-called VDSL (very high speed digital subscriber line) method and the VDSL2 method. In this context, transmission rates of 13 MBit/s up to 55 MBit/s are achieved in the downstream (from the switching center to the subscriber) and transmission rates between 1.5 MBit/s and 15 MBit/s are achieved in the upstream (from the subscriber to the switching center) in the case of VDSL on the conventional telephone line between the first switching center on the office side (mostly a so-called DSLAM—digital subscriber line access multiplexer) and subscriber line. With VDSL2, it is even possible to achieve transmission rates of up to 100 MBit/s (downstream and upstream). The information or data are transmitted here in accordance with the so-called frequency division multiplex method, i.e. according to a method in which the frequency bands for downstream and upstream are separated. Thus, for example, two downstream and two upstream frequency bands are in each case basically provided in VDSL which together extend over the frequency spectrum from 135 kHz to 12 MHz. In VDSL2, up to three frequency bands are currently in each case used for downstream and upstream which, in addition to the spectrum utilized with VDSL, are also distributed over the frequencies from 12 MHz to 30 MHz.
The abovementioned physical connection between a subscriber and the switching center (i.e. the conventional telephone line) consists in most cases of a pair of wires, mostly of a twin copper wire. Since several of these subscriber lines can lie combined bundled within a cable, interference and disturbances occur between the wires carried in a cable bundle. These disturbances, or the interfering noises occurring on the lines, become particularly noticeable at the receiver inputs of the transmission system. Such disturbances are also called crosstalk and are frequency-dependent. In addition, a distinction is made here between the so-called near end crosstalk (NEXT) and the far end crosstalk (FEXT).
The near end crosstalk, which is also called transverse attenuation, is a measure of the suppression of the crosstalk between two adjacent pairs of wires at the beginning/end of the cable. The near end crosstalk specifies how strongly the signal of a pair of wires is induced into another pair of wires at the location of the in-feed, that is to say at the transmitter. In contrast, the far end crosstalk relates to the far end of the line, that is to say the receiver end of the transmission link. A signal fed into a wire is reduced by the cable attenuation at the end of the line. The interfering noise which occurs due to crosstalk of this signal to another wire at the receiver end is called far end crosstalk.
In xDSL transmission systems which operate in accordance with the abovementioned frequency division multiplex method (such as, for instance, VDSL2), the disturbances generated by FEXT are essentially determining for a maximum achievable data rate, that is to say for the so-called performance. The performance can be reduced considerably by the FEXT interference noise especially in the VDSL2 transmission technology which, as mentioned above, also uses high frequency bands (up to 30 MHz) for the transmission.
The problem of the FEXT interference noises is additionally increased if the individual subscriber lines have different distances to the office side. If, for example, all subscriber lines were to be supplied with the same transmission power, then shorter lines, that is to say those lines, the subscribers of which are arranged closer to the office side, would have a distinctly higher received signal power at the office side than the signals on longer lines which have been attenuated over a longer distance. These stronger signal powers on shorter subscriber lines would greatly interfere with weaker signals on longer subscriber lines which would lead to greatly different performances of the individual subscriber lines.
To solve this problem, the possibility of the so-called “upstream power back-off” (UPBO) is provided in ITU standard G.993.2, chapter 7.2.1.3 and in ITU G.997.1. Using the UPBO mechanism, the upstream transmission power is controlled in such a manner that all upstream signals have signal levels that are as similar as possible at the receiving side. This ensures that all subscribers can be provided approximately with a similar data rate on the corresponding subscriber line. The UPBO improves the spectral compatibility of the subscriber lines.
To carry out the UPBO mechanism, the steps explained in the text which follows are carried out during a training phase when setting up the connection. At the beginning of the training, the transmission unit at the office side (DSLAM) conveys to the subscriber unit predeterminable standard signals. These can contain, for example, information about the signal power with which these signals were sent out and also specifications with respect to the power with which upstream signals sent from the subscriber should arrive at the DSLAM. By means of this information, the subscriber unit determines in a first step line parameters of the communication link. In this context, for example, the signal strength of the received signals is detected and, by means of the information about the original transmission power of the signals, the attenuation within the subscriber line and, respectively, a corresponding value for the electrical length of the line (usually designated as kl0), is estimated. Using this knowledge, the transmission power can be established in the subscriber unit in such a manner that the signals should reach the office side as accurately as possible with the power required by the DSLAM. Values for this required received power can be predetermined by the operator of the communication network, for example via network management settings. At the end of the training phase, the value representing the transmission power is finally also transmitted from the subscriber unit to the DSLAM.
As described above, the UPBO mechanism attempts to place the received power of all subscriber lines on an as similar as possible a value in order to obtain the least possible effects due to FEXT interference. However, this concept overlooks the fact that subscribers with shorter subscriber lines can easily operate with a comparatively somewhat higher transmission power (and thus have a higher received power and better performance) without causing additional FEXT interference on the comparatively longer subscriber lines: since the interference caused by FEXT at the end of a line is also dependent on the total length of the subscriber line previously to be bridged, subscriber units with shorter distance from the office side can actually transmit with higher power without exceeding the permitted limit of generated FEXT interference. A higher transmission power on the subscriber side alone does not automatically generate FEXT interference which is too great at the office side on short subscriber lines; due to the shortness of the line (i.e. greater FEXT attenuation, less crosstalk), such FEXT interference is no stronger, even with higher transmission power, than, for example, in the case of lower transmission powers on longer subscriber lines.
To achieve a higher performance for individual subscribers (for those who are located closer to the office side), their transmission power can therefore be raised to a higher level without other subscribers being disturbed beyond the permitted extent.
(A further possibility would be, for instance, not to increase the transmission power of the shorter subscriber lines but to utilize the resultant better performance of the longer subscriber lines. Thus, for example, the possibility would exist that, when considering the DSLAM subscriber loop as a whole, the performance can be increased balanced for all subscribers instead of improving the performance for individual subscriber lines.)
For the case considered above that the subscribers who are closer with respect to the office side operate with higher transmission power and thus obtain a better performance, however, there is one problem: such an increase in transmission power for individual subscriber lines is not provided in the standards quoted above and particularly the calculations according to the standards, described in greater detail below, of the transmission power in the individual subscriber units.